CDG Customs, Car Rental and Drive to Normandy

Personally I would not do the Normandy trip. Normandy is big -- 50 miles east to west plus the stuff behind the beaches. While it is easy to see with a vehicle, there is distance and time. On returning from Normandy I would drop the car at the airport or somewhere in Paris as a car is a problem in Paris. Use local transportation to see the sights.

Everything Frank said. Normandy is not as small or as close to Paris as many people from this side of the Atlantic imagine. Your aggressive plan would also heavily depend on how well you sleep on a plane and how much jet lag would make you a dangerous driver. Plus, there is so much to do and see in Paris in that limited time. As a direct answer, it is apt to take at least an hour and possibly more to get from your plane to the rental car office, and a good hour+ just to get out of the rush hour traffic on the Paris periphery roads—another two or more hours to get from there to Bayeux. Possibly much more for both. I truly adore Normandy (ten visits so far and returning this summer), but I would strongly recommend you put off this visit until you can give it the time it deserves.

You don't say where in Normandy. It's a large region, much more than the spreadout D-Day landing beaches.
But if you are going to drive to an airport from Normandy, Orly is closer. I'd allow 3 hours to return the car and check in, go through security and get to your gate at CDG but 2 hours at Orly, the smaller airport.

If this quick trip was important to me, I would take the train from Paris to Normandy and pick up a car there for the day, return it the next, and take the train back to Paris.

And if I’m flying business class, I’m taking taxis both ways between CDG to Gare St-Lazare.

ScandiFans plan makes the most sense, but ONLY if you know exactly what you want to see and do on arrival in whatever part of Normandy you plan to visit. And you need to consider how jet lag usually affects you. You would need to catch an early morning train back to Paris, keeping in mind that the car rental agency may not open until 0800 or 0830. You would be lucky to arrive back in Paris by late morning. And you would need to be at CDG by 1500 at the very latest for a 1700 departure. Its a bad idea, giving you very little time in Normandy to make all that effort worthwhile. Better to stay in Paris and leave Normandy for a future trip when you can give it the time it deserves.

What's your destination in southern France? Do you need to be there for dinner? There are evening flights from Caen to Nice and Toulouse via Lyon, as well as afternoon flights to Marseille on some days.

With those, assuming you don't need to be in the south for dinner on day 2, you could conceivably start with Paris, take a train to Caen once you've seen the Eiffel Tower, rent a car and spend the late afternoon and next morning/early afternoon around the D Day beaches, return the car at the airport and fly out.

If your Normandy destination is Mont St Michel, it's even more feasible: Mont St Michel is a quick visit and Rennes airport has more flights. See what you want to see in Paris, take a train to Rennes (only 1.5 hours!), rent a car, drive to the Mont St Michel and spend the night, and drive back to Rennes airport whenever, possibly swinging by Fougères castle if you have extra time (but be wary of Rennes traffic).

Where do you want to go in Normandy ?

It is over 11,000 square miles in size.